Chapter 19 Variants of Preexcitation
Variants of Preexcitation (Atypical Bypass Tracts)
A working definition of an atypical bypass tract (BT) is a conduction pathway that bypasses all or part of the normal conduction system but is not a rapidly conducting pathway connecting atrium and ventricle near the mitral or tricuspid annulus. Thus, pathways that connect the atrium to the His bundle (HB), the atrioventricular node (AVN) to the His-Purkinje system (HPS) or the ventricle, or the HPS to the ventricle fit into this designation (Fig. 19-1).
“Mahaim Fibers”
In 1937, during pathological examination of the heart, Mahaim and Benatt identified islands of conducting tissue extending from the HB into the ventricular myocardium. These fibers were called Mahaim fibers or fasciculoventricular fibers.1–3 This description was subsequently expanded to include connections between the AVN and the ventricular myocardium (nodoventricular fibers). Later, it was recognized that BTs could arise from the AVN and insert into the right bundle branch (RB; nodofascicular fibers).2,3 This classification for Mahaim fibers persisted until evidence suggested that the anatomical substrate of tachycardias with characteristics previously attributed to nodoventricular and nodofascicular fibers is actually atrioventricular (AV) and atriofascicular BTs with decremental conduction properties (i.e., conduction slows at faster heart rates) (see Fig. 19-1). Although these BTs are sometimes collectively referred to as “Mahaim fibers,” the use of this term is discouraged because it is more illuminating to name the precise BT according to its connections. In this chapter, these BTs are referred to as atypical BTs to differentiate them from the more common (typical) rapidly conducting AV BTs that result in the Wolff-Parkinson-White (WPW) syndrome.4
“Mahaim Tachycardia”
The term Mahaim tachycardia is used to describe the typical constellation of electrophysiological (EP) features that characterize the unusual form of reentrant tachycardia using an atypical BT, without implication about the underlying anatomical cause. It should be noted that, because the term was originally applied to an anatomical finding and subsequently (incorrectly) applied to physiology that matched what would be expected from this anatomy, it has given rise to more confusion than understanding. Hence, the use of the term Mahaim tachycardia should generally be discouraged; instead, one should simply describe the physiological characteristics of the tachyarrhythmia.5
Types of Atypical Bypass Tracts
Long Decrementally Conducting Atrioventricular and Atriofascicular Bypass Tracts
These BTs comprise the majority (80%) of atypical BTs; their atrial insertion site is in the right atrial (RA) free wall.6,7 These BTs tend (84%) to cross the tricuspid annulus in the lateral, anterolateral, or anterior region. They extend along the right ventricular (RV) free wall to the region where the moderator band usually inserts at the apical third of the RV free wall, inserting into the distal part of the RB (atriofascicular BT) or into the ventricular myocardium close to the RB (long decrementally conducting AV BT). These BTs are functionally similar to the normal AV junction, with an AVN-like structure leading to a His bundle (HB)–like structure. In essence, those BTs function as an auxiliary conduction system parallel to the normal conduction system (AVN–HPS). Similar to the normal AVN, these BTs demonstrate decremental conduction (related to the slow rate of recovery of excitability) and Wenckebach-type block in response to rapid atrial pacing and are sensitive to adenosine. The conduction delay in these BTs has been localized to the intraatrial portion of the BT (the AVN-like portion), whereas the interval from the inscription of the BT potential at the tricuspid annulus and the onset of ventricular activation (BT-V interval) remains constant.4–6,8,9
Short Decrementally Conducting Atrioventricular Bypass Tracts
These BTs are analogous to decrementally conducting concealed BTs responsible for the permanent form of junctional reciprocating tachycardia (PJRT; see Chap. 18) in that they bridge the AV rings and insert proximally into ventricular myocardium in close proximity to the AV annulus.7,10 These BTs primarily arise from the RA free wall, but can also arise from the posterior or septal region. Left-sided BTs with decremental conduction characteristics have rarely been described. Although these BTs demonstrate decremental conduction and Wenckebach-type block in response to rapid atrial pacing, they do not consistently appear to be responsive to adenosine, which suggests that their structure is not composed of AVN-like tissue.10
Nodoventricular and Nodofascicular Bypass Tracts
Nodoventricular BTs arise in the normal AVN and insert into ventricular myocardium near the AV junction.7 Nodofascicular BTs arise in the normal AVN and insert into the RB. These BTs are sensitive to adenosine, probably because of their AVN connection.5
Arrhythmias Associated with Atypical Bypass Tracts
Atypical BTs in patients with clinical arrhythmias have the following characteristics: (1) unidirectional (anterograde-only) conduction (with rare exceptions); (2) long conduction times; and (3) decremental conduction (i.e., cycle length [CL]-dependent slowing of conduction).
Atypical BTs comprise 3% to 5% of all BTs. The incidence is slightly higher (6%) in patients presenting with supraventricular tachycardia (SVT) with a left bundle branch block (LBBB) morphology.7 Multiple BTs occur in 10% of patients with atypical BTs. In some cases, a typical, rapidly conducting AV BT can mask the presence of an atypical BT, which only becomes apparent after ablation of the typical BT. Dual AVN pathways or multiple BTs occur in 40% of patients with atypical BTs. Atypical BTs can also be associated with Ebstein anomaly.
Supraventricular Tachycardias Requiring An Atrioventricular Bypass Tract for Initiation and Maintenance
Antidromic atrioventricular reentrant tachycardia (AVRT) can use the atypical BT anterogradely and the HPS-AVN retrogradely. Antidromic AVRT can also use the atypical BT anterogradely and a second AV BT retrogradely. In the latter case, the AVN can participate as an innocent bystander mediating anterograde or retrograde fusion. Because these atypical BTs almost always conduct anterogradely only, they cannot mediate orthodromic AVRT, but can mediate antidromic AVRT or can be an innocent bystander during other SVTs (e.g., atrioventricular nodal reentrant tachycardia [AVNRT]). However, they can coexist with typical rapidly conducting AV BTs. AVRTs using a nodoventricular or nodofascicular BT as the anterograde limb generally use a second AV BT as the retrograde limb of the reentrant circuit.5 Right free wall atriofascicular BTs capable of both anterograde and retrograde conduction that participate in both antidromic and orthodromic AVRT have rarely been reported.11
Electrocardiographic Features
Normal Sinus Rhythm
During normal sinus rhythm (NSR), the ECG shows normal QRS or minimal preexcitation in most patients with atypical BTs. Subtle preexcitation can be suspected by the absence of the normal septal forces (small q waves) in leads I, aVL, V5, and V6 and the presence of an rS complex in lead III in the setting of a narrow QRS.12 The degree of preexcitation depends on the relative conduction time over the AVN and BT. Maneuvers that prolong conduction over the AVN (e.g., atrial pacing, vagal maneuvers, or drugs) to a greater degree than BT conduction will increase the degree of preexcitation. Because atypical BTs exhibit decremental conduction, increasing the atrial pacing rate results in prolongation of the P-delta interval. In contrast, in the setting of typical rapidly conducting AV BTs, the P-delta interval remains relatively constant regardless of the degree of preexcitation; whereas prolonging the AVN conduction time results in more preexcitation. The P-delta interval remains constant or exhibits mild prolongation because conduction over the typical BT displays less decrement than does the AVN.5,7,12–14
Preexcited Qrs Morphology
For atriofascicular and nodofascicular BTs, the QRS is relatively narrow (133 ± 10 milliseconds), and its morphology is classic for typical LBBB with a QRS axis between 0 and –75 degrees and a late precordial R/S transition zone (at V4 or V5, and sometimes V6). However, for long decrementally conducting AV BTs, the QRS is relatively wider (166 ± 26 milliseconds) and the LBBB pattern is less typical (with broad initial R in V1). The QRS is even wider and the LBBB pattern is less typical with nodoventricular and decrementally conducting short AV BTs than that with atriofascicular or long decrementally conducting AV BTs.5,12,14
Supraventricular Tachycardias
Arrhythmias associated with atypical BTs are associated with an LBBB pattern and, most often, in the setting of long decrementally conducting AV and atriofascicular BTs, left axis deviation on the surface ECG (Fig. 19-2). There are several ECG features that suggest (although are not diagnostic of) atypical BTs as the cause of an SVT with LBBB pattern. These include (1) QRS axis between 0 and –75 degrees, (2) QRS duration of 150 milliseconds or less, (3) R wave in lead I, (4) rS complex in lead V1, and (5) precordial R wave transition in lead V4 or later.5,12,14
Electrophysiological Testing
Baseline Observations During Normal Sinus Rhythm
Atrial Pacing and Atrial Extrastimulation during Normal Sinus Rhythm
Progressively shorter atrial pacing CLs or atrial extrastimulus (AES) coupling intervals produce decremental conduction in both the atypical BT and, to a greater degree, the AVN (Fig. 19-3).12 Consequently, the atrial–His bundle (AH) interval increases, the QRS morphology gradually shifts to a more preexcited LBBB morphology, and the AV (A-delta) interval increases. However, the AV (A-delta) interval increases to a lesser degree than the AH interval. This is in contrast to the setting of typical rapidly conducting AV BTs, in which the AV (A-delta) interval remains constant despite prolongation of the AH interval and exaggeration of the degree of preexcitation, because the A-delta interval represents conduction time over the BT. Typical AV BTs maintain constant conduction time during different pacing rates and AES coupling intervals—that is, nondecremental conduction.5
With progressively shorter atrial pacing CLs or AES coupling intervals, the HV interval decreases as the His potential becomes progressively inscribed into the QRS (usually within the first 5 to 25 milliseconds after the onset of the QRS). The His potential eventually becomes activated retrogradely as the wavefront travels anterogradely down the BT and then retrogradely up the RB to the HB (see Fig. 19-3). When the His potential is lost within the QRS, it is unclear whether anterograde AV conduction continues to propagate over the HB or block has occurred.7
At the point of maximal preexcitation, the AV (A-delta) interval continues to prolong with more rapid pacing because of the decremental conduction properties of the BT, and the His potential–QRS relationship remains unaltered because the HB is activated retrogradely until block in the BT occurs. The fixed ventricular–His bundle (VH) interval, despite shorter pacing CLs or AES coupling intervals, suggests that the BT inserts into or near the distal RB at the anterior free wall of the RV with retrograde conduction to the HB. Whenever the VH interval is less than 20 milliseconds, insertion into the RB (i.e., atriofascicular or nodofascicular BT) is likely. On the other hand, with long decrementally conducting AV BTs, which insert into the ventricular myocardium close to the RB, the VH interval approximates the HV interval minus the duration of the His potential (because the His potential is activated retrogradely).5,7
For short decrementally conducting BTs, the HB is activated anterogradely, and retrograde conduction to the HB is only seen following AV block or during antidromic AVRT. Decremental conduction (progressive prolongation of the AV interval) and Wenckebach-type block develop in the BT. The conduction delay in these BTs is localized to the intraatrial portion of the BT; the interval from the inscription of the BT potential at the tricuspid annulus to the onset of ventricular activation (BT-V interval) remains constant.5
Effects of Adenosine
Adenosine produces conduction delay in most atypical BTs except for short decrementally conducting AV BTs. The conduction delay has been localized to the intraatrial portion of the BT; the interval from the inscription of the BT potential at the tricuspid annulus and the onset of ventricular activation remains constant.7,13 When adenosine administration slows AVN conduction, an increase in the degree of preexcitation is noted in all types of BTs (as long as adenosine does not block the BT).
Induction of Tachycardia
Initiation by Atrial Extrastimulation or Atrial Pacing
As noted, progressively shorter atrial pacing CLs (especially from the RA) result in progressive AV (A-delta) interval prolongation and a greater degree of preexcitation until maximal. Often, once maximal preexcitation has been achieved, cessation of pacing is followed by preexcited SVT. Progressively shorter AES coupling intervals similarly result in progressive AV (A-delta) interval prolongation and a greater degree of preexcitation until maximal. When anterograde AVN conduction fails but conduction persists over the BT, the HPS-AVN can be activated retrogradely to initiate antidromic AVRT.7
The sudden appearance of preexcitation associated with a “jump” from the fast to the slow AVN pathway with a His potential inscribed before ventricular activation or with a VH interval of less than 10 milliseconds strongly favors AVNRT. Although a slowly conducting atriofascicular BT that becomes manifest with a jump to the slow AVN pathway cannot be excluded, a consistent pattern of dual pathway dependence and an HV relationship too short to be retrograde from the distal RB would be unlikely.7 Induction of AVNRT with AES is almost always associated with a dual pathway response, which may not be seen if the impulse conducts anterogradely over the BT and captures the HB before it is activated by the impulse traversing the slow AVN pathway anterogradely. In other cases, a jump can be seen so that the anterograde His potential follows the QRS with a typical AVN echo to initiate SVT, analogous to 1:2 conduction initiating antidromic AVRT.
Initiation by Ventricular Extrastimulation or Ventricular Pacing
Ventricular pacing can initiate SVT in 85% of cases. Initiation is almost always associated with retrograde conduction up a relatively fast AVN pathway, followed by anterograde conduction down a slow pathway, which is associated with preexcitation. The anterograde slow pathway can be a BT (i.e., antidromic AVRT) or a slow AVN pathway (i.e., AVNRT with an innocent bystander BT). During induction of the SVT by ventricular pacing at a CL similar to the tachycardia CL or by a VES that advances the His potential by a coupling interval similar to the H-H interval during the SVT, the His bundle–atrial (HA) interval following the ventricular stimulus is compared with that during the SVT—an HA interval that is longer with ventricular pacing or VES initiating the SVT than that during the SVT suggests AVNRT. This occurs despite the fact that the H-H interval of the VES (i.e., the interval between the His potential activated anterogradely by the last sinus beat to the His potential activated retrogradely by the VES initiating the SVT) exceeds the H-H interval during the SVT. Because the AVN usually exhibits greater decremental conduction with repetitive engagement of impulses than in response to a single impulse at a similar coupling interval, the more prolonged the HA with the initiating ventricular stimulus, the more likely the SVT is AVNRT. If the SVT uses the BT for anterograde conduction, the HA interval during ventricular pacing or the VES initiating the SVT, at a comparable coupling interval as the tachycardia CL, should have the same HA interval as during the SVT.
